2.7.23 Secondary PumpsSecondary pumps in a primary/secondary chilled water pumping system are used to distribute the chilled water to the loadsfrom the primary production loop. The primary/secondary pumping system is used to decouple one piping loop fromanother hydronically. In this case, the primary pump maintains a constant flow through the chillers, allowing the secondarypumps to vary flow, increase control and save energy.If the primary/secondary design concept is not used and a variable volume system is designed, the chiller cannot shed itsload properly when the flow rate drops far enough or too quickly. The low evaporator temperature safety then trips thechiller, requiring a manual reset. This situation is common in large installations, especially when two or more chillers areinstalled in parallel.2.7.24 The VLT SolutionWhile the primary/secondary system with 2-way valves improves energy and system control, using adjustable frequencydrives increases the energy savings and control potential further.With the proper sensor location, the addition of adjustable frequency drives allows the pumps to match their speed to thesystem curve instead of the pump curve.This eliminates wasted energy and most of the over-pressurization, two-way valves can be subjected to.As the monitored loads are reached, the two-way valves close down, increasing the differential pressure measured acrossthe load and two-way valve. As this differential pressure starts to rise, the pump is slowed to maintain the control head alsocalled setpoint value. This setpoint value is calculated by adding the pressure drop of the load and the two-way valve underdesign conditions.NOTICE!When running multiple pumps in parallel, they must run at the same speed to increase energy savings, either withindividual dedicated adjustable frequency drives, or one adjustable frequency drive running multiple pumps in parallel.FrequencyconverterFrequencyconverterCHILLERCHILLER33P130BB454.10Figure 2.16 Adjustable Frequency Drives used with Secondary Pumps in a Primary/Secondary Pump SystemIntroduction Design GuideMG16C122 Danfoss A/S © Rev. 2014-01-13 All rights reserved. 332 2
